The acceptor-donor pair recombination of beryllium-treated sapphires

Recently, the concept of Fe3+-Ti4+ mixed acceptor states in the energy band gap of sapphire has been proposed as the cause of blue coloration. The blue color in sapphire is faded and turns yellow by beryllium heat treatment. It has been revealed as the color center in Be-treated yellow sapphires. In this work, the mechanism of such phenomena was explained and discussed. The experiment was set by preparing sapphire samples from Songea, Tanzania, into four color groups (purplish blue, light blue, light yellow with dark zone, and light green). One sample from each group, altogether four, has been heat-treated with beryllium. The colors of Be-treated samples show fading of blue coloration and adding the yellow component to the samples. The analyses and spectroscopic testing were conducted on both the untreated and treated samples, in each group which consisted of one natural and one Be-treated sample. The content of color-related trace elements such as Be, Mg, Ti, Fe, and Cr was measured by the LA-ICP-MS. The oxidation state of Fe [III] was confirmed by X-ray absorption spectroscopy. Each color group showed quite similar UV-Vis-NIR absorption spectra for the natural and the Be-treated ones, regardless of differences in their color and treatment. However, their UV-Vis-NIR polychromatic excitation spectra were different, leading to the possible detection of blue color suppression due to the recombination of Fe3+-Ti4+ mixed-acceptor states by the Fe3+-Be2+ mixed donor states for Be-treated samples. Hence, the acceptor-donor pair recombination has been proposed for the blue color fading in beryllium-treated sapphire samples.